Apparatus and method for measuring the air flow component and water vapor component of air/water vapor streams flowing under vacuum

ABSTRACT

The inventive method for measuring the mass of the gas component and the mass of the water vapor component of a gas/water vapor source stream flowing in a conduit under reduced pressure comprises storing a correlation of relative saturation of water in said gas and the output electrical signal of a relative humidity sensor. Next, a correlation of the flow rate of the gas component of a flowing gas/water vapor reference stream and the output electrical signal of a temperature-compensated flow sensor as a function of water vapor density is stored. A correlation of maximum water vapor saturation in the gas as a function of temperature also is stored. An electrical signal of a temperature sensor in contact with said source stream then is generated. An output electrical signal of a relative humidity sensor in contact with said source stream also is generated. An output electrical signal of a temperature-compensated flow sensor in contact with said source stream further is generated. The relative humidity output signal is compared with said stored relative saturation correlation to derive a relative saturation value which value is multiplied by the maximum water vapor saturation correlation for the temperature determined from the temperature sensor signal in order to derive the mass density of the water vapor component of said source stream Finally, the flow sensor signal is compared with said stored gas flow rate correlation at derived water vapor mass density to derive the flow rate of the gas component of said source stream. Also disclosed is the apparatus for carrying out the method. Also disclosed is a method and apparatus to incorporate a pressure sensor along with these three primary sensors to provide other correlations to provide redundancy to the relative humidity sensor in the event of its failure and to provide indications of the existence of water mist contamination of the source stream.

BACKGROUND OF THE INVENTION

The present invention relates to the measurement of flow rates ofgas/water vapor mixtures through conduit and more particularly to themeasurement of the gas component and the water vapor component undervacuum flow conditions in the conduit.

The measurement of steam and water flow rates in steam power plants(both conventional and nuclear) is a required operation at a variety ofjunctures in the various steam and water circuits comprising the steampower plant. Referring to FIG. 1, steam produced within a boiler vessel10 is taken through steam line 12 and applied to steam turbine 14.Exhaust steam 16 from turbine 14 is condensed in a condenser 18 andreturned via line 20 and pump 22 to pressure vessel 10 via line 24 asfeedwater. Cooling water is passed through a multiplicity of tube, suchas tube 17, connected between headers 19 of condenser 18 via line 26 bypump 28. Finally, vacuum pump 30 maintains reduced pressure in condenser18 via line 32 with line 34 vented to atmosphere. It is desirable tomeasure the flow rate of air that enters condenser 18 via leaks to thislow pressure region and removed by pump 30, with flow sensor 36 locatedin line 32.

Over the years, industry has recognized the importance to minimize theback-pressure at the exhaust of the steam turbines where such lowpressure condensers are employed. As exhaust pressure rises, more fuelis required to generate additional steam to maintain turbine outputpower. An increase in the back-pressure decreases plant operatingefficiency. In the electrical power industry, millions of dollars peryear per generating plant can be lost due to unnecessary rise in turbineexhaust pressure. A manageable cause for increased back-pressure is airleakage into the evacuated end of the turbine and its exhaust steamcondenser.

To remove air and other non-condensable gases that enter the condenserspace, a vacuum pump, air injector, or some other contrivance isemployed. For systems most commonly employed, a line or conduitgenerally is installed between the condenser and the pump within whichthese gases are conveyed. They enter the air removal equipment generallyat a pressure in the range of about 1 to 6 inches of mercury absolutewhere they are compressed to atmospheric pressure and released to theenvironment.

In addition to non-condensable gases passing through the conduit, anamount of water vapor is contained which generally is determined by theabsolute pressure within the condenser and the temperature of coolingwater passing through the condenser tubes. The partial pressure of thewater vapor generally is related to the temperature of the coolingwater. This pressure represents the minimum obtainable back pressureavailable to the turbine and generally is related to local environmentalconditions. Pressures higher than this which are caused by air leaksrequire attention because this added pressure component can bediminished by maintenance procedures.

Flow instruments available on the market have limitations which precludetheir use for measurement of air flow in the conduit at low pressure.The mass flow rate is below the low end sensitivity of most instruments.Further, the variable concentration of combined air and water vapor flowcannot properly be separated such that air only is measured.

The most common method of measuring air leaks is to physically observethe height of a float in a variable area flow meter or the pressure dropacross an orifice in a differential pressure flow meter, either of whichis connected to the exhaust end of the vacuum pump. In addition to beinginconvenient, high inaccuracy is present using these methodologies. Theair is contaminated with liquid water due, in part, to condensation ofthe pumped water vapor from the condenser being raised to atmosphericpressure and also from pump designs which use a water seal, both ofwhich adds buoyancy to the float or partial closure of the orifice.Another limitation of these methodologies is that the measurementrequires plant personnel to observe and record at intervals which isinconvenient and costly to the plant operator. Further, without anoutput signal that can be recorded in real time, the measurement cannotbe correlated in time with other events which could be used to identifysources of leaks. Also, because of the measurement location, themeasurement cannot be used to determine if the air flow results from afault pump seal or an air leak into the condenser space.

BROAD STATEMENT OF THE INVENTION

The present invention broadly is addressed to a method, system, andapparatus for determining the amount of non condensable gas, e.g. air,flowing within a conduit under low pressure conditions in the presenceof varying amounts of water vapor. Although the invention may find otherapplications such as, for example, the measurement of air flow in dryersoperating at various absolute pressures as may be found, for example, inthe food or paper industry processing, the invention will be describedfor the specific application of measuring non-condensable gas flow inthe suction line of air removal equipment found in steam turbine powergenerating equipment (both nuclear and conventional fuel plants).

Although a principal element of the invention is a flow velocity sensingdevice, the invention includes the application, in vacuum, of a watervapor density sensor including temperature compensation which can beused to compensate the non-condensable gas flow sensor signal for theamount of water vapor content. Additionally, a pressure sensing devicealso is described as providing indication of distinctly different statesof operation which may occur within the condenser and for determiningthe flow rate of the water vapor component. The pressure sensor signalalso may be used for flow signal manipulation should the water vaporsensor become non-operational. The system employs proven flow,temperature, and pressure sensing devices along with common water vaporsensing devices found by testing to perform in a vacuum environment. Asa consequence, a practical and low cost measuring apparatus is realizedsuitable for quantifying air leaks into the vacuum regions of steamturbines. The apparatus employs direct measurement of water vaporcontent in the flowing stream rather than relying on the assumption thatequilibrium state physical laws are represented by indirect measurementsas may be found in other systems.

The inventive method for measuring the mass of the gas component and themass of the water vapor component of a gas/water vapor source streamflowing in a conduit under reduced pressure comprises storing acorrelation of relative saturation of water in said gas and the outputelectrical signal of a relative humidity sensor. Next, a correlation ofthe flow rate of the gas component of a flowing gas/water vaporreference stream and the output electrical signal of atemperature-compensated flow sensor as a function of water vapor densityis stored. A correlation of maximum water vapor saturation in the gas asa function of temperature also is stored. An electrical signal of atemperature sensor in contact with said source stream then is generated.An output electrical signal of a relative humidity sensor in contactwith said source stream also is generated. An output electrical signalof a temperature-compensated flow sensor in contact with said sourcestream further is generated. An output electrical signal of a pressuresensor in contact with the source stream is generated. The relativehumidity output signal is compared with said stored relative saturationcorrelation to derive a relative saturation value which value ismultiplied by the maximum water vapor saturation correlation for thetemperature determined from the temperature sensor signal in order toderive the mass density of the water vapor component of said sourcestream The flow sensor signal is compared with said stored gas flow ratecorrelation at derived water vapor mass density to derive the flow rateof the gas component of said source stream. Finally, the measuredpressure, air mass flow rate, partial pressure water vapor as a functionof water vapor density and conduit geometry are manipulated to obtainthe water mass flow rate. Knowing these, the mass flow rate of the watervapor and air can be summed and by comparing these data with pumpcapacity curves, the plant operator also has a means by which pumpdegradation can be measured.

The inventive apparatus which implements the foregoing method comprises:(a) memory in which is stored the correlation of relative saturation ofwater in said gas and the output electrical signal of a relativehumidity sensor; (b) memory in which is stored the correlation of theflow rate of the gas component of a flowing gas/water vapor referencestream and the output electrical signal of a temperature-compensatedflow sensor as a function of water vapor density; (c) memory in which isstored a correlation of maximum water vapor saturation in the gas as afunction of temperature; (d) a temperature sensor in thermal contactwith the source stream which generates an electrical signalrepresentative of the temperature of said source stream; (e) a relativehumidity sensor in contact with said source stream which generates anelectrical signal representative of the relative humidity in said sourcestream; (f) a temperature-compensated flow sensor in contact with saidsource stream which generates an electrical signal representative of theflow rate of said source stream; (g) a comparator which compares thesignal from relative humidity sensor (e) with said stored correlation inmemory (a) to derive a relative saturation value which value ismultiplied by the maximum water vapor saturation correlation in memory(c) for the temperature determined from temperature sensor (d) in orderto derive the mass density of the water vapor component of said sourcestream; and (h) a comparator which compares the electrical signal fromflow sensor (f) with said stored correlation in memory (b) for thederived water vapor density (g) to derive the flow rate of the gascomponent of said source stream.

The relative humidity or water vapor sensor output signal is calibratedfor determining the relative saturation of water vapor in a gas/watervapor mixture held under reduced pressure and various temperatures bythe steps of: (a) in a gas/water vapor mixture of known compositionwhich is less than saturation and which is held under reduced pressureand various temperatures, recording the electrical signal from arelative humidity sensor which signal corresponds to the known watervapor density; (b) determining the water vapor saturation density fromknown "properties of saturated steam" at said varying temperatures for apractical range of water temperatures held under reduced pressure; (c)at each temperature, dividing the known water vapor density from step(a) by the saturation water vapor density of step (b) to generate therelative water vapor saturation at varying temperatures and pressures;and (d) finally correlating the output electrical signal in step (a) tothe relative saturation generated in step (c).

Advantages of the present invention include the recognition thatconventional water vapor (relative humidity) sensors can be used invacuum lines for the determination of mass density and relativesaturation of water vapor in a flow under vacuum. Another advantage isthe ability to separately determine both gas (air) flow rate and watervapor flow rate in a mixture flow under vacuum. A further advantage isthat the inventive apparatus conveniently can be combined with aprocessor (computer) for additional capability and flexibility. Theseand other advantages will be readily apparent from the disclosureherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of a steam turbine generator plantwhere steam is generated to energize a turbine for the generation ofelectricity.

FIG. 2 is a simplified schematic representation of the components usedin forming the novel apparatus which measures the mass of gas and watervapor components of the gas/water vapor stream; and

FIG. 3 is a simplified schematic of a water concentration calibrationapparatus used to calibrate the output electrical signal of a relativehumidity sensor for use in vacuum lines;

FIG. 4 graphically plots a correlation of the water vapor concentratedvoltage of the relative humidity sensor to the water content (expressedin mass per unit volume) as a function of temperature;

FIG. 5 is a graphical plot of the normalized water vapor concentrationvoltage of the relative humidity sensor versus relative saturation ascalculated from the results displayed in FIG. 4;

FIG. 6 is a schematic of the air flow measurement configuration used tocorrelate the flow rate of the gas component of a flowing gas/watervapor reference stream and the output electrical signal of atemperature-compensated flow sensor as a function of temperature andwater vapor concentration;

FIG. 7 is a graphic plot of the data generated using the apparatus inFIG. 6 where volumetric air flow is plotted versus output voltage of thetemperature-compensated flow sensor at various water vaporconcentrations; and

FIG. 8 is a simplified perspective view of a water vapor sensor housedin a chamber for use when greater than 100% relative saturation or steamconditions are encountered.

The drawings will be described in detail in connection with thefollowing description.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus and method for measuring the mass flow rate of the gascomponent and the mass of the water component of a gas/water vaporsource stream flowing in a conduit under reduced pressure are unique forthis purpose, but also are unique within a system wherein alarm limitscan be set for monitoring the flow conditions within the conduit, sensedconditions beyond normal limits triggering an alarm enabling theoperator to effect corrective action. The apparatus of the presentinvention is seen in simplified schematic form in FIG. 2 and is seen toinclude temperature and flow sensor 38 which has an output signalcarried via line 40 to processor 42 which contains memory and acomparator, water vapor sensor 44 whose output signal in line 46 isconnected to processor 42, and pressure sensor 48 whose output signal inline 50 similarly is connected to processor 42. Processor 42conveniently can be a personal computer, such as a Compaq Portable IIcomputer (Compaq Computer Corp.). Signal line 52 from processor 42publishes a variety of information which has been gathered from theindicated sensors. For example, from temperature and flow sensor 38 andwater vapor sensor 44, which sensors have been properly calibrated inaccordance with the precepts of the present invention, air flow rate 54,water vapor density 56, and temperature 60 can be published. Of course,with sensor 48, pressure 58 additionally is known. Since air mass flowrate 54, water vapor density 56, and pressure 58 are known, then watervapor mass flow rate 65 is determinable. Another parameter of interestto the power plant operator is the presentation of the mass ratio ofwater vapor to air 67, computed by dividing the mass flow rate of watervapor 65 by the mass flow rate of air 54. Finally, a variety of alarmlimits can be published such as, for example, low flow flag 62 and highflow flag 64, both conditions being outside the air mass flow ratelimits of calibration, to name but two of the variety of alarm limitsfor which the inventive system has the capability.

The hardware components disclosed herein generate signals that can beprocessed to derive indicia or parameters (such as described above) ofthe source stream by a variety of mathematical manipulations andcomputations. Preferred signal manipulations will be described in detailherein by way of illustration of the present invention and not by way oflimitation.

As an initial step in the present invention, water vapor sensor 44(common relative humidity sensors not specifically designed for vacuumapplications) needs to be calibrated so that the water content, orrelative saturation, can be determined directly from the electricalsignal output from water vapor sensor 44. In this regard, reference ismade to FIG. 3 wherein the water concentration calibration apparatus isset forth in a simplified schematic drawing. It will be observed thatchamber 66 (1000 ml flask), tubing 68, water vapor sensor 70(Panametrics HybridCap sensor, Panametrics, Inc., Waltham, Mass.)powered by a Kepco PS ATE 150 power supply (Kepco Inc., Flushing, N.Y.),and pressure sensor 72 (Ashcroft K2, Ashcroft, Dresser Industries,Stratford, Conn.) are housed within isothermal tank 74. Read-out 76 isconnected via line 78 to water vapor sensor 70 and pressure read-out andpower source 80 is connected to pressure sensor 72 via line 82. Vacuumpump 84 (Welch Model No. 1400) with air outlet line 86 is connected vialine 88 and valve 90 to chamber 66 for evacuating air from chamber 66 toestablish a vacuum. Sensor 72 with associated readout 80 ensures theconstancy of the vacuum drawn when valve 90 is closed.

Once the vacuum has been established and observed to remain constant byobserving readout 80 with valve 90 closed, a known amount of liquidwater placed in syringe 92 can be injected into chamber 66 throughtubing 68. Because of apparent physisorption by the glass flask andother internal surfaces of water vapor, the amount of water vapor inchamber 66 was determined from the pressure increase (water vaporpressure) by dint of the water injection. Isothermal oven 74 insures atemperature such that only the vapor phase or state of water ispermissible within chamber 74 and establishes the temperature at whichwater vapor saturation density will be computed. At varying temperaturesand varying water content established in chamber 66, readings from watervapor sensor 70 were taken and recorded. To insure the water vaporsensor was immune to air partial pressure, the vacuum pressure wasvaried three times by injecting 50 cc of atmospheric pressure air intochamber 66 through tubing 66 using another syringe. The water vaporreadings remaining unchanged. This is so since the volume of the tankand the volume of water therein were constant with the water alwaysbeing in the vapor state. The signal from sensor 70 in line 78 wasrecorded as set forth in Table 1 and plotted in FIG. 4 versus watercontent established in chamber 66 at the various temperatures.

                                      TABLE 1                                     __________________________________________________________________________    SATURATION SENSOR CALIBRATION                                                                 Calculated                                                                            Water Vapor                                           Measured                                                                             Measured Water                                                                         Water Vapor                                                                           Saturation                                                                            Percent                                                                             Sensor                                  Temperature                                                                          Vapor Pressure                                                                         Density Density Saturation                                                                          Output                                  Degrees F.                                                                           in. Hg absolute                                                                        lbm/cubic ft.                                                                         lbm/cubic ft.                                                                         %     Volts                                   __________________________________________________________________________    129.7  0.0066   0.00001 0.00628 0.1   1.620                                   111.5  0.009    0.00001 0.00390 0.3   1.615                                   77.8   0.007    0.00001 0.00149 0.7   1.621                                   75.2   0.007    0.00001 0.00137 0.8   1.619                                   69.1   0.012    0.00002 0.00113 1.7   1.620                                   129.2  0.449    0.00063 0.00621 10.1  1.636                                   116.9  0.400    0.00057 0.00451 12.7  1.645                                   104.6  0.287    0.00042 0.00323 13.0  1.644                                   118.0  0.629    0.00090 0.00465 19.3  1.657                                   87.4   0.272    0.00041 0.00198 20.7  1.6701                                  129.3  0.985    0.00138 0.00621 22.2  1.663                                   117.0  0.748    0.00107 0.00453 23.6  1.670                                   104.9  0.624    0.00091 0.00325 28.0  1.676                                   129.2  1.304    0.00182 0.00621 29.4  1.692                                   58.6   0.144    0.00023 0.00077 29.5  1.684                                   73.4   0.253    0.00039 0.00130 30.1  1.681                                   59.0   0.156    0.00025 0.00079 31.4  1.676                                   75.6   0.297    0.00046 0.00139 32.9  1.697                                   129.3  1.469    0.00205 0.00622 33.0  1.686                                   78.1   0.328    0.00050 0.00150 33.5  1.689                                   116.8  1.144    0.00163 0.00450 36.3  1.693                                   117.9  1.275    0.00182 0.00467 39.3  1.695                                   87.8   0.527    0.00079 0.00200 39.7  1.710                                   105.7  0.943    0.00137 0.00333 41.2  1.704                                   129.6  1.964    0.00275 0.00626 43.9  1.706                                   117.0  1.498    0.00214 0.00452 47.3  1.714                                   106.8  1.252    0.00182 0.00343 53.0  1.726                                   129.7  2.442    0.00341 0.00628 54.3  1.726                                   118.4  1.904    0.00271 0.00470 57.7  1.725                                   117.0  1.866    0.00267 0.00453 58.8  1.734                                   78.4   0.621    0.00095 0.00151 62.9  1.735                                   129.7  2.944    0.00411 0.00629 65.4  1.745                                   88.3   0.884    0.00133 0.00203 65.6  1.745                                   106.7  1.552    0.00226 0.00342 65.9  1.749                                   117.6  2.187    0.00312 0.00460 67.9  1.751                                   129.7  3.327    0.00465 0.00628 74.0  1.761                                   117.9  2.461    0.00351 0.00464 75.7  1.761                                   107.2  1.816    0.00264 0.00347 76.2  1.766                                   117.5  2.503    0.00357 0.00459 77.8  1.766                                   129.8  3.696    0.00516 0.00630 82.0  1.776                                   106.6  1.933    0.00281 0.00342 82.3  1.773                                   88.4   1.122    0.00169 0.00203 82.9  1.771                                   117.3  2.659    0.00380 0.00457 83.1  1.774                                   118.8  2.887    0.00411 0.00475 86.5  1.779                                   117.7  3.137    0.00448 0.00461 97.0  1.794                                   117.1  3.208    0.00458 0.00454 100.8 1.799                                   __________________________________________________________________________

The water vapor saturation density was obtained from tables "propertiesof saturated steam" found in any textbook for the temperature beingmeasured (e.g., Handbook of Chemistry and Physics, 47th edition, pageE-11 et seq., The Chemical Rubber Company, 1966; and Perry, et al.,Chemical Engineers' Handbook, Fifth Edition, McGraw-Hill Book Company,1973), the disclosures of which are expressly incorporated herein byreference. Such tables also can include, for example, water vaporpartial pressure, specific volume, enthalpy, and entropy data at varioustemperatures. Note the data of Table 1 is ordered for increasing percentsaturation. The data for all temperature calibration runs presented inTable 1 is plotted graphically in FIG. 5 showing percent saturationversus sensor output voltage. The curve is nearly identical to whatmight be expected at atmospheric pressure for the same sensor and showsthis device is suitable for vacuum applications. With the calibration ofwater vapor sensor 70 complete, electrical signals from it can bedirectly converted into relative saturation values which can be storedin memory in processor 42 which additionally can be programmed forinterpolation for values not in storage or an algorithm can be employed.It should be noted that additional common relative humidity sensors (notdesigned for vacuum operation) were calibrated under vacuum with similarresults. These sensors included a MiniCap 2 (Panametrics, Inc.), anEMD-2000 (Phys-Chem Scientific Corp.), and a Series 691 (PhilipsComponents, Inc.). Thus, it appears that commercial relative humiditysensors do function under vacuum. Assuming reproducibility of readingsof various models of sensors, calibration of each relative humiditysensor should not be necessary, but for, perhaps, a single test pointfor each unit to be run to confirm this assumption. It should beunderstood, however, that relative humidity sensors specially designedfor operation in vacuum could be used to advantage in the presentinvention and the calibration scheme set forth herein would probably notbe necessary.

Next, flow sensor 38 in FIG. 2 can be calibrated similarly using theequipment configuration depicted in FIG. 6. Chambers 94 and 96 (250 mlflasks) in FIG. 6 are connected by line 98. Chamber 94 is fitted withmagnetic stirrer assembly 100 (model 120M stirrer, Fisher ScientificCorp.), heater 102 and temperature sensor 104 which are connected totemperature controller 106. Water 108 is housed in chamber 94. Line 98is fitted with pressure sensor assembly 108 while chamber 96 is fittedwith water vapor sensor assembly 110. Air is metered through variableare flow meter 112 (Model F1-1805, Omega Engineering, Inc.) and passedinto chamber 94 via line 114 which is fitted with valve 116 suitable foradjusting air flow. Air/water vapor mixture is withdrawn from chamber 94through chamber 96 via line 118 and passed through flow meter 120(Rheotherm LFI-100-TU1/8 flow instrument, Intek, Inc., Westerville,Ohio). The electrical signal from flow meter 120 passes via line 122into readout 124. Vacuum pump 126 (Model 1400, Welch Co.) provides themotive force for the air and air/water vapor flows via line 128 which isconnected to flow meter 120 and thence exhausted through line 130.Isothermal chamber 132 was used to maintain the external temperature ofchambers 94 and 96 with temperature sensor assembly 134 providing thereadout of such temperature.

In order to compensate for temperature variations of the air flowthrough flow meter 120, air only runs (no water in chamber 94) wereconducted at 70° and 117° F. and the output signals recorded as setforth in Table 2 below.

                                      TABLE 2                                     __________________________________________________________________________             Output Signal       Signal                                                          117° F. Ambient                                                                      Difference due                                   Air Flow Rate                                                                          70° F.                                                                              Temperature                                                                          to Temperature                                   Standard CC                                                                            Measured                                                                            Measured                                                                             Compensated                                                                          (Measured                                        per min  Signal                                                                              Signal Signal Signals)                                         (SCCM)   Volts Volts  Volts  mV                                               __________________________________________________________________________    1872     7.211 7.041  7.253  170                                              1713     7.332 7.148  7.360  184                                              1516     7.470 7.258  7.470  212                                              1323     7.593 7.388  7.600  205                                              1143     7.740 7.542  7.754  198                                              967.6    7.928 7.729  7.941  199                                              799.8    8.180 7.983  8.195  197                                              568.4    8.554 8.315  8.527  239                                              338.3    9.060 8.808  9.020  252                                              116.0    9.978 9.702  9.914  276                                              Average Signal Difference (mV)                                                                             213                                              Temperature Compensation (mV/°F.) 70° F. Reference                                           4.5                                              __________________________________________________________________________

These data demonstrate that an error correction of about 213/(117-70) or4.5 mV/°F. can be applied to the hot air signals in order to provide allsignals normalized to about room temperature, thus eliminatingtemperature effects experienced by flow meter 120.

Using the equipment configuration in FIG. 6, known air flow rates wereadmitted into the system at various water vapor concentrations inchamber 96 (as determined from calibrated water vapor sensor assembly110) at various temperatures. The data recorded is set forth in Table 3below.

                                      TABLE 3                                     __________________________________________________________________________    FLOW RATE vs. WATER VAPOR DENSITY                                             65° F. Ambient   98° F. Ambient                                                                            112° F. Ambient             Dry Air                                                                            Temperature                                                                          Relative                                                                           Water  Temperature                                                                          Relative                                                                           Water  Temperature  Water                 Flow Compensated                                                                          Sat- Vapor  Compensated                                                                          Sat- Vapor  Compensated                                                                          Relative                                                                            Vapor                 Rate Flow Signal                                                                          uration                                                                            Density                                                                              Flow Signal                                                                          uration                                                                            Density                                                                              Flow Signal                                                                          Saturation                                                                          Density               SCCM Volts  %    Lbm/Cu. ft.                                                                          Volts  %    Lbm/Cu. ft.                                                                          Volts  %     Lbm/Cu.               __________________________________________________________________________                                                            ft                    1872 7.095  75.8 .000747                                                                              6.918  57.3 .00115 6.519  63.9  .00253                1323 7.399  75.2 .000742                                                      1143                    7.303  58.4 .00117 6.826  56.9  .00225                967.6                                                                              7.682  76.9 .000759                                                      779.8                   7.612  58.9 .00118 7.033  60.4  .00239                568.4                                                                              8.204  82.7 .000816                                                                              8.125  59.8 .00120                                    338.3                                                                              8.613  83.1 .000820                                                                              8.125  60.2 .00120 7.294  61.1  .00242                225.2                   8.333  60.2 .00121                                    125.6                                      7.475  61.2  .00242                116  9.293  85.4 .000842                                                                              8.522  61.9 .00124                                    67.1                    8.616  61.9 .00124                                    42.0 9.563  82.5 .000814                                                                              8.650  62.7 .00126 7.566  60.8  .00241                Average Water Vapor     Average Water Vapor                                                                              Average Water Vapor                Density (Lbm/Cu. ft.) = .000791                                                                       Density (Lbm/Cu. ft.) = .00121                                                                   Density (Lbm/Cu. ft.) =            __________________________________________________________________________                                               .00240                         

The above-tabulated data also is depicted graphically in FIG. 7. Note,that the air only calibration results represented by line 136 at twotemperatures, 70° and 117° F. also is shown. Line 146 corresponds to awater vapor density in air of about 0.0008 lbs/ft³ ; and lines 148 and150 correspond, respectively, to an average value of 0.0012 and 0.0024lb/ft³, respectively. Again, these data can be stored in memory inprocessor 42. Alternatively, one could generate an empirical formula tomodel the data in Table 3 and FIG. 7 and use such formula to determinethe air mass flow rate and mass of water vapor flowing in a conduitusing the signals from the water vapor sensor temperature sensor and theflow sensor.

A determination of the water vapor flow rate in the conduit can beobtained by multiplying the water vapor density by the velocity of theflowing gas (air)/water vapor mixture and by the cross-sectional area ofthe conduit (which area was previously determined and entered intomemory). The water vapor density is derived from the measuredtemperature and relative water vapor saturation, as described above. Thevelocity of the stream is determined by dividing the derived air flowrate (e.g., in units of SCFM) by the cross-sectional are of the conduitand multiplying by the ratio of the standard or normal atmosphericpressure of 29.9 inches Hg absolute divided by the stream air partialpressure. The stream air partial pressure can be found by subtractingthe water vapor partial pressure from the measured total pressurederived from the output signal of the pressure sensor. The water vaporpartial pressure is found from the stored correlation of saturated watervapor density and water vapor partial pressure as a function of streamtemperature using the determined water vapor density.

Another important parameter to be determined is the water vapor mass toair mass ratio. To derive this ratio the processor is instructed todivide the water mass flow rate by the air mass flow rate and displaythe result. Since the air mass flow rate is displayed in units of SCFM,the conversion to lbs/minute simply is performed by multiplying SCFM bythe density of dry air at standard conditions which has been stored inmemory.

As to appropriate flow sensors for use in the inventive apparatus,virtually any flow sensor (or flow meter) can be used from simple flowrestriction or orifice flow sensors to sophisticatedmicroprocessor-based flow meters. The preferred flow sensor is disclosedin U.S. Pat. No. 4,255,968, the disclosure of which is expresslyincorporated herein by reference. The flow sensor of choice desirablyhas been "temperature-compensated" (such as described above) or is"temperature independent".

It is entirely conceivable that the water vapor in conduit 32 (FIG. 1)subjected to measurement could be over 100% (supersaturated) or containa water mist due to a variety of reasons. Thus, appropriate water vaporsensor 44 (FIG. 2) should be capable of operating near 100% relativehumidity, such as a Phys-Chem EMD-2000 micro relative humidity sensor(Phys-Chem Scientific Corp., New York, N.Y.). As to reasons for thiscondition, line 32 may be located in an environment where the ambienttemperature is below the cooling water temperature of condenser 18.Also, should the extraction suction line shroud surrounding a section ofcooling tubes within condenser 18 develop openings or otherwise not beproperly sealed, steam can enter the line directly without passing overthe cooling tubes.

When steam mist in present in line 32, water vapor sensor 44 would besaturated and unable to provide an output in relation to water content.Referring to FIG. 8, water vapor sensor 44 is housed within residencechamber 138 which provides temperature control by heater 140 so that thetemperature is greater than in line 32 by some amount, say up to 5° F.Space 142 optionally may be packed or a screen may surmount aperture 144to "demist" the air entering chamber 138. Since relative humiditydecreases by about 3% for each 1° F. increase in temperature, the airwithin chamber 138 would be "dried" by heater 140 permitting ameasurement of the water vapor concentration at the higher temperature,resulting in a determination of the higher water content. The higherindicated water vapor content at the flow sensor then would be used tocorrect the flow sensor reading of the flow sensor to obtain the airflow rate, computed water vapor density, and water to air mass ratio. Ifthe water vapor sensor cannot effect a reading with the chosen 5° F.temperature excursion, then an alarm should be activated as a steam,leak or other failure mode may be indicated.

Alternative, if a mist detector were used when mist was present, theremight not be a need to measure water vapor density with the relativesaturation sensor. The water vapor concentration (density) would be atthe value of saturation determined from temperature and the "steamtable" correlation.

When steam mist is present in line 32 as described above, theconfiguration for the relative saturation sensor of FIG. 8 could permitits output signal to no longer be saturated but not be indicative of thewater mist content. Mist for some flow sensors could cause a signalvariation above that defined for water vapor content. In this case,measurement of the water mist content would be useful for the purpose ofcorrecting the flow sensor signal in a manner described above using therelative saturation sensor for water vapor content.

An example of such a sensor would be an optical device where a lightbeam would undergo scattering or otherwise be attenuated in relation towater mist content similar to clouds in the sky. A light intensitysensor output signal would be calibrated as a function of mist densityand used to correct the flow signal of mist sensitive gas/water vaporflow sensors. Another method can be visualized where a heated surfaceexposed to the mist containing steam and another turned away from directimpact of the steam could be utilized. The temperature differentialbetween the two surfaces would be a measure of the mist content due tothe cooling effect of the impacting water droplets. This method would besimilar to sensing methods disclosed in U.S. Pat. No. 4,255,968 butemployed for a different purpose.

Other alarm limits can be set, for example, based on loss of vacuum asmeasured by the pressure sensor indicative of loss of pump efficiency ora leak in the pumping system, unexpectedly high temperature with highpressure which may indicate condenser binding, unexpectedly lowtemperature with high pressure and saturated water vapor which also mayindicate condenser binding, and the like.

Finally, should the water vapor sensor fail, the efficiency of thecondenser can be assumed to be high and the gas/water vapor streamtemperature can be assumed to be the same as the condenser tubetemperature at the stream conduit shroud interface. With theseassumptions, along with the measured total pressure for the stream adetermination of the partial pressure of the water vapor, and hence itsmass density could be calculated.

It will be appreciated that a wide variety of combinations of operatingparameters can be envisioned for the inventive apparatus, especiallywhen the capabilities of a computer are incorporated into the design.The description herein is by way of illustration and not by way oflimitation. All citations referred to herein are expressly incorporatedherein by reference.

I claim:
 1. Method for measuring the flow rate of the gas component andthe mass density of water vapor component of a gas/water vapor sourcestream flowing in a conduit under reduced pressure, which comprises thesteps of:(a) storing a correlation of relative saturation of water vaporin said gas and the output electrical signal of a relative humiditysensor; (b) storing a correlation of the flow rate of the gas componentof a flowing gas/water vapor reference stream and the output electricalsignal of a temperature-compensated flow sensor as a function of watervapor density; (c) storing a correlation of maximum water vaporsaturation in the gas as a function of temperature; (d) generating anelectrical signal of a temperature sensor in contact with said sourcestream; (e) generating an output electrical signal of a relativehumidity sensor in contact with said source stream; (f) generating anoutput electrical signal of a temperature-compensated flow sensor incontact with said source stream; (g) comparing said signal (e) with saidstored correlation (a) to derive a relative saturation value which valueis multiplied by the maximum water vapor saturation correlation (c) forthe temperature determined from signal (d) in order to derive the massdensity of the water vapor component of said source stream; and (h)comparing said signal (f) with said stored correlation (b) at derivedwater vapor mass density (g) to derive the flow rate of the gascomponent of said source stream.
 2. The method of claim 1, wherein saidrelative humidity sensor is calibrated by the steps of:(a1) in agas/water vapor mixture held under sub-atmospheric pressure, fixing thewater vapor concentration at varying values up to saturation andrecording the electrical signal from said relative humidity sensor whichsignal corresponds thereto; (a2) at each temperature, dividing the watervapor concentration from step (a1) by the saturation value of step (c)to generate the relative saturation of water vapor; and (a3) correlatingthe electrical signal of said relative humidity sensor to the relativesaturation under sub-atmospheric pressure generated in step (a2).
 3. Themethod of claim 1, wherein said conduit is a vacuum suction line from acondenser which receives steam from a steam turbine.
 4. The method ofclaim 1, wherein said gas is air.
 5. The method of claim 1, whichincludes the step of:(i) generating an output signal of a pressuresensor in contact with the source stream in said conduit.
 6. The methodof claim 5, wherein the water vapor mass flow rate in the conduit isdetermined by the steps of:(j1) storing the cross-sectional area of theconduit; (j2) storing the partial pressure of water vapor as a functionof water vapor density; (j3) storing the standard atmospheric pressureof said gas; (j4) determining the partial pressure of the gas componentby subtracting the stored water vapor partial pressure (j2) at the watervapor density derived in (g) from total pressure signal (i); (j5)determining the source stream rate by dividing the stream flow rate fromstep (h) by the stored area (j1), and then multiplying by the ratio ofstandard atmospheric pressure (j3) divided by the gas partial pressurein step (j4); and (j6) determining the water vapor flow rate bymultiplying the mass density of water vapor derived in step (g) by thestream rate determined in step (j5) and by the stored area (j1).
 7. Themethod of claim 6, wherein a measured pressure above an alarm limittriggers an alarm indicative thereof.
 8. The method of claim 6, whichincludes the steps of:(k) storing the density of air at standardconditions; (l) multiplying the stored density of air (k) by the airflow rate (h) in units of SCFM to derive the mass flow rate of air; and(m) dividing the mass flow rate of water vapor (j6) by the mass flowrate of air (h) to determine the mass ratio of water vapor to air. 9.Apparatus for measuring the flow rate of the gas component and the massdensity of the water vapor component of a gas/water vapor source streamflowing in a conduit under sub-atmospheric pressure, which comprises:(a)memory in which is stored the correlation of relative saturation ofwater vapor in said gas and the output electrical signal of a relativehumidity sensor; (b) memory in which is stored the correlation of theflow rate of the gas component of a flowing gas/water vapor referencestream and the output electrical signal of a temperature-compensatedflow sensor as a function of water vapor density; (c) memory in which isstored a correlation of maximum water vapor saturation in the gas as afunction of temperature; (d) a temperature sensor in thermal contactwith the source stream which generates an electrical signalrepresentative of the temperature of said source stream; (e) a relativehumidity sensor in contact with said source stream which generates anelectrical signal representative of the relative saturation in saidsource stream; (f) a temperature-compensated flow sensor in contact withsaid source stream which generates an electrical signal representativeof the flow rate of said source stream; (g) a comparator which comparesthe signal from relative humidity sensor (e) with said storedcorrelation in memory (a) to derive a relative saturation value whichvalue is multiplied by the maximum water vapor saturation correlation inmemory (c) for the temperature determined from temperature sensor (d) inorder to derive the mass density of the water vapor component of saidsource stream; and (h) a comparator which compares the electrical signalfrom flow sensor (f) with said stored correlation in memory (b) toderive the flow rate of the gas component of said source stream.
 10. Theapparatus of claim 9, which additionally comprises:(i) a pressure sensorin contact with the source stream in said conduit for generating anelectrical signal representative of the pressure of the source stream insaid conduit.
 11. The apparatus of claim 10, wherein additionallycomprises:(j) memory in which is stored the cross-sectional area of theconduit; (k) memory in which is stored standard atmospheric pressure ofsaid gas; (l) memory in which is stored the partial pressures of watervapor as a function of water vapor density; (m) calculator for: (1)subtracting the stored water vapor partial pressure from memory (l) fromtotal pressure measured in (i) for determining the partial pressure ofthe gas component; (2) dividing the stream flow rate from comparator (h)by the cross-sectional area in memory (j), and then multiplying by theratio of standard atmospheric pressure in memory (k) divided by the gaspartial pressure determined in (m1) for determining the source streamrate; and (3) multiplying the mass density of water vapor derived incomparator (g) by the stream rate determined in (m2) and by the storedcross-sectional area in memory (j) for determining the water vapor flowrate.
 12. The apparatus of claim 10, which additionally comprises:(m)memory in which is stored a pressure alarm limit; (n) a comparator thatcompares the signal from pressure sensor (i) with the stored alarmlimit; and (o) an alarm that is activated when comparator (n) determinesthat the signal from pressure sensor (i) is greater than the storedalarm limit.